Cinnamal-nitrile dyes for laser recording element

ABSTRACT

A laser recording element comprising a support having thereon a dye layer comprising an image dye dispersed in a polymeric binder, said dye layer having an infrared-absorbing material associated therewith, and wherein said dye layer also contains a cinnamal-nitrile UV-absorbing dye.

This invention relates to use of cinnamal-nitrile UV dyes in a single-sheet laser recording element.

In recent years, thermal transfer systems have been developed to obtain prints from pictures which have been generated electronically from a color video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to the cyan, magenta and yellow signals. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271, the disclosure of which is hereby incorporated by reference.

Another way to thermally obtain a print using the electronic signals described above is to use a laser instead of a thermal printing head. In such a laser transfer system, the donor sheet includes a material which strongly absorbs at the wavelength of the laser. When the donor is irradiated, this absorbing material converts light energy to thermal energy and transfers the heat to the dye in the immediate vicinity, thereby heating the dye to its vaporization temperature for transfer to a receiver. The absorbing material may be present in a layer beneath the dye and/or it may be admixed with the dye. The laser beam is modulated by electronic signals which are representative of the shape and color of the original image, so that each dye is heated to cause volatilization only in those areas in which its presence is required on the receiver to reconstruct the color of the original object. Further details of this process are found in GB 2,083,726A, the disclosure of which is hereby incorporated by reference.

In another mode of imaging using a laser beam, a laser recording element with a dye layer composition comprising an image dye, an infrared-absorbing material, and a binder coated onto a substrate is imaged from the dye side. The energy provided by the laser drives off the image dye and other components of the dye layer at the spot where the laser beam impinges upon the element. In "laser removal" imaging, the laser radiation causes rapid local changes in the imaging layer, thereby causing the material to be removed from the layer. Usefulness of such a laser recording element is largely determined by the efficiency at which the imaging dye can be removed on laser exposure. The transmission Dmin value is a quantitative measure of dye clean-out: the lower its value at the recording spot, the more complete is the attained dye removal.

U.S. Pat. 5,256,506, in column 12, describes various non-black body absorbers such as dicinnamalacetone and benzophenone derivatives which are supposed to be useful in graphic arts masks. However, there is a problem with dicinnamalacetone in that under accelerated light fade conditions, the loss in UV density is pronounced, as will be shown by comparative tests hereafter. There also is a problem with benzophenone in that it has insufficient UV absorption to be an effective graphic arts mask, as will be shown by comparative tests hereafter.

It is an object of this invention to provide a UV-absorbing dye which will have both an acceptable level of UV absorption and improved stability to UV density loss from light exposure. It is another object of this invention to provide a single-sheet process which does not require a separate receiving element.

These and other objects are achieved in accordance with the invention which comprises a laser recording element comprising a support having thereon a dye layer comprising an image dye dispersed in a polymeric binder, the dye layer having an infrared-absorbing material associated therewith, and wherein the image dye is a cinnamal-nitrile UV-absorbing dye.

In a preferred embodiment of the invention, the cinnamal-nitrile UV-absorbing dye has the following structure: ##STR1## wherein R₁ represents hydrogen or an alkyl group having from 1 to about 6 carbon atoms;

R₂ represents cyano or a carboxylic ester group; and

R₃ -R₇ each independently represents hydrogen, cyano, alkoxy, an alkyl group having from 1 to about 6 carbon atoms, nitro, a carboxylic ester group or an electron-withdrawing moiety, such as sulfonyl, trifluoromethyl, halo, etc.

The cinnamal-nitrile UV-absorbing dye may be used in an amount of from about 0.05 to about 1.0 g/m² of element.

In a preferred embodiment of the invention, in the above formula, R₁ represents hydrogen, R₂ represents cyano or COOCH₃, and R₃ or R₅ represents methoxy.

Specific examples of UV dyes according to the above formula include the following: ##STR2##

A visible image dye can also be used in the laser recording element employed in the invention provided it can be removed by the action of the laser. Especially good results have been obtained with dyes such as anthraquinone dyes, e.g., Sumikaron Violet RS® (product of Sumitomo Chemical Co., Ltd.), Dianix Fast Violet 3R-FS® (product of Mitsubishi Chemical Industries, Ltd.), and Kayalon Polyol Brilliant Blue N-BGM® and KST Black 146® (products of Nippon Kayaku Co., Ltd.); azo dyes such as Kayalon Polyol Brilliant Blue BM®, Kayalon Polyol Dark Blue 2BM®, (products of Nippon Kayaku Co., Ltd.); direct dyes such as Direct Dark Green B® (product of Mitsubishi Chemical Industries, Ltd.) and Direct Brown M® (product of Nippon Kayaku Co. Ltd.); acid dyes such as Kayanol Milling Cyanine 5R® (product of Nippon Kayaku Co. Ltd.); basic dyes such as Sumiacryl Blue 6G® (product of Sumitomo Chemical Co., Ltd.), and Aizen Malachite Green® (product of Hodogaya Chemical Co., Ltd.); ##STR3## or any of the dyes disclosed in U.S. Pat. Nos. 4,541,830, 4,698,651, 4,695,287, 4,701,439, 4,757,046, 4,743,582, 4,769,360, and 4,753,922, the disclosures of which are hereby incorporated by reference. The above dyes may be employed singly or in combination. The dyes may be used at a coverage of from about 0.05 to about 1 g/m² and are preferably hydrophobic.

Another embodiment of the invention relates to a process of forming a dye image comprising imagewise-heating, by means of a laser, the recording element described above, the laser exposure taking place through the dye side of the element and causing dye to be removed imagewise to obtain the dye image in the recording element.

The laser recording elements of this invention can be used to obtain medical images, reprographic masks, printing masks, etc. The image obtained can be a positive or a negative image. The dye removal process can generate either continuous (photographic-like) or halftone images.

The invention is especially useful in making reprographic masks which are used in publishing and in the generation of printed circuit boards. The masks are placed over a photosensitive material, such as a printing plate, and exposed to a light source. The photosensitive material usually is activated only by certain wavelengths. For example, the photosensitive material can be a polymer which is crosslinked or hardened upon exposure to ultraviolet or blue light but is not affected by red or green light. For these photosensitive materials, the mask, which is used to block light during exposure, must absorb all wavelengths which activate the photosensitive material in the Dmax regions and absorb little in the Dmin regions. For printing plates, it is therefore important that the mask have high blue and UV Dmax. If it does not do this, the printing plate would not be developable to give regions which take up ink and regions which do not.

By use of this invention, a mask can be obtained which has enhanced light stability for making multiple printing plates or circuit boards without mask degradation.

Any polymeric material may be used as the binder in the recording element employed in the invention. For example, there may be used cellulosic derivatives, e.g., cellulose nitrate, cellulose acetate hydrogen phthalate, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, cellulose triacetate, a hydroxypropyl cellulose ether, an ethyl cellulose ether, etc., polycarbonates; polyurethanes; polyesters; poly(vinyl acetate); polystyrene; poly(styrene-co-acrylonitrile); a polysulfone; a poly(phenylene oxide); a poly(ethylene oxide); a poly(vinyl alcohol-co-acetal) such as poly(vinyl acetal), poly(vinyl alcohol-co-butyral) or poly(vinyl benzal); or mixtures or copolymers thereof. The binder may be used at a coverage of from about 0.1 to about 5 g/m².

In a preferred embodiment, the polymeric binder used in the recording element employed in the process of the invention has a polystyrene equivalent molecular weight of at least 100,000 as measured by size exclusion chromatography, as described in U.S. Pat. No. 5,330,876, the disclosure of which is hereby incorporated by reference.

A barrier layer may be employed in the laser recording element of the invention if desired, as described in U.S. Pat. No. 5,459,017, the disclosure of which is hereby incorporated by reference.

To obtain a laser-induced image according to the invention, an infrared diode laser is preferably employed since it offers substantial advantages in terms of its small size, low cost, stability, reliability, ruggedness, and ease of modulation. In practice, before an infrared laser can be used to heat a recording element, the element must contain an infrared-absorbing material, such as cyanine infrared-absorbing dyes as described in U.S. Pat. No. 5,401,618 or other materials as described in the following U.S. Pat. Nos. 4,948,777, 4,950,640, 4,950,639, 4,948,776, 4,948,778, 4,942,141, 4,952,552, 5,036,040, and 4,912,083, the disclosures of which are hereby incorporated by reference. The laser radiation is then absorbed into the dye layer and converted to heat by a molecular process known as internal conversion. Thus, the construction of a useful dye layer will depend not only on the hue, transferability and intensity of the image dyes, but also on the ability of the dye layer to absorb the radiation and convert it to heat. The infrared-absorbing dye may be contained in the dye layer itself or in a separate layer associated therewith, i.e., above or below the dye layer. Preferably, the laser exposure in the process of the invention takes place through the dye side of the recording element, which enables this process to be a single-sheet process, i.e., a separate receiving element is not required.

Lasers which can be used in the invention are available commercially. There can be employed, for example, Laser Model SDL-2420-H2 from Spectra Diode Labs, or Laser Model SLD 304 V/W from Sony Corp.

The dye layer of the laser recording element of the invention may be coated on the support or printed thereon by a printing technique such as a gravure process.

Any material can be used as the support for the recording element of the invention provided it is dimensionally stable and can withstand the heat of the laser. Such materials include polyesters such as poly(ethylene naphthalate); polysulfones; poly(ethylene terephthalate); polyamides; polycarbonates; cellulose esters such as cellulose acetate; fluorine polymers such as poly(vinylidene fluoride) or poly(tetrafluoroethylene-co-hexa-fluoropropylene); polyethers such as polyoxymethylene; polyacetals; polyolefins such as polystyrene, polyethylene, polypropylene or methylpentene polymers; and polyimides such as polyimide-amides and polyether-imides. The support generally has a thickness of from about 5 to about 200 μm. In a preferred embodiment, the support is transparent.

The following example is provided to illustrate the invention.

EXAMPLE

The following materials were employed in this example: ##STR4##

A 100 μm thick poly(ethylene terephthalate) support was coated with a laser dye ablation layer consisting of 0.22 g/m² infrared dye IR-1, 0.60 g/m² nitrocellulose, and either 0.27 g/m² of Control-1 or 0.86 mmol/m² of Control-2, Control-3, or any of the compounds A-J of the invention coated from tetrahydrofuran.

The stability of the resulting dye layers was measured using an X-Rite Densitometer (Model 361T, X-Rite Corp.) by the percent change in UV density between a covered and uncovered sample after exposure to four hours of 50 kLux sunshine. The following results were obtained:

    ______________________________________                                                Target  UV                                                                     Lay-    Density  UV Density                                                                              Percent                                              down    COV-     UN-      UV                                            Dye    (g/m.sup.2)                                                                            ERED     COVERED  Change λmax                            ______________________________________                                         Control 1                                                                             0.25    2.506    0.85     -66    364                                    Control 2                                                                             0.25    3.282    0.352    -89    364, 410                               Control 3                                                                             0.16    0.456    0.422     -7    400                                    A      0.16    1.768    0.813    -54    357                                    B      0.18    2.301    0.83     -64    394                                    C      0.21    1.117    0.563    -50    341                                    D      0.17    1.792    0.965    -46    352                                    E      0.20    1.207    0.566    -53    345                                    F      0.20    2.353    0.816    -65    366                                    G      0.22    2.888    0.861    -70    372                                    H      0.21    1.536    0.882    -43    357                                    I      0.19    1.111    0.552    -50    347                                    J      0.18    1.485    0.544    -63    346                                    ______________________________________                                    

The above results show that the dyes of the invention are comparable, if not superior, to the control dyes in their UV density loss; they are more resistant to fading in UV light then control dyes C1 and C2. Control dye C3 has insufficient UV density to serve as effective graphic arts mask material.

It is likewise seen from the data presented in the Table that the class of UV-absorbing dyes of the present invention encompasses compounds with a wide range of UV absorption peaks. This is of advantage in that it will be possible to "tailor" a desired absorption wavelength range or a specific wavelength by either a mixture of selected compounds or by using a specific dye of this class.

Printing

Samples of the above example were laser written on a drum printer using a laser diode print head, where each laser beam has a wavelength range of 830-840 nm and a nominal power output of 550 mW at the film plane.

The drum, 53 cm in circumference, was rotated at varying speeds and the imaging electronics were activated to provide adequate exposure. The translation stage was incrementally advanced across the recording element by means of a lead screw turned by a microstepping motor, to give a center-to-center line distance of 10.58 mm (945 lines per centimeter or 2400 lines per inch). The laser energy impinges on the recording element in accordance with the electronic information of the image. The energy provided by the laser drives off the image dye and other components of the dye layer at the spot where the laser beam impinges upon the element. The removed dye and other effluents are collected by suction. The measured total power at the focal plane was 550 mW per channel maximum. A useful image was obtained.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 

What is claimed is:
 1. A laser recording element comprising a support having thereon a dye layer comprising an image dye dispersed in a polymeric binder, said dye layer having an infrared-absorbing material associated therewith, and wherein said image dye is a cinnamal-nitrile UV-absorbing dye, with a λ-max in the UV of 300 -400 nm, wherein said cinnamal-nitrile UV-absorbing dye has the following structure: ##STR5## wherein R₂ represents hydrogen or an alkyl group having from 1 to about 6 carbon atoms;R₂ represents cyano or a carboxylic ester group; and R₃ -R₇ each independently represents hydrogen, cyano, alkoxy, an alkyl group having from 1 to about 6 carbon atoms, nitro, a carboxylic ester group or an electron-withdrawing moiety.
 2. The element of claim 1 wherein R₁ represents hydrogen, R₂ represents cyano or COOCH₃, and R₃ or R₅ represents methoxy.
 3. The element of claim 1 wherein said infrared-absorbing material is a dye which is contained in said dye layer.
 4. A process of forming a dye image comprising imagewise-heating, by means of a laser, a recording element comprising a support having thereon a dye layer comprising an image dye dispersed in a polymeric binder, said dye layer having an infrared-absorbing material associated therewith, said laser exposure taking place through the dye side of said element and causing dye to be removed imagewise to obtain said dye image in said recording element, wherein said image dye is a cinnamal-nitrile UV-absorbing dye, with a λ-max in the UV of 300 -400 nm, wherein said cinnamal-nitrile UV-absorbing dye has the following structure: ##STR6## wherein R₁ represents hydrogen or an alkyl group having from 1 to about 6 carbon atoms;R₂ represents cyano or a carboxylic ester group; and R₃ -R₇ each independently represents hydrogen, cyano, alkoxy, an alkyl group having from 1 to about 6 carbon atoms, nitro, a carboxylic ester group or an electron-withdrawing moiety,whereby a dye image is obtained having improved stability to UV density loss from light exposure.
 5. The process of claim 4 wherein R₁ represents hydrogen, R₂ represents cyano or COOCH₃, and R₃ or R₅ represents methoxy.
 6. The process of claim 4 wherein said infrared-absorbing material is a dye which is contained in said dye layer. 